This invention concerns polyolefin coated support material for thermal dye transfer processes, as well as a process for producing same.
The system of thermal dye transfer (dye diffusion thermal transfer "D2T2") makes it possible to reproduce an electronically generated image in the form of a "hard copy". The principle of thermal dye transfer consists of the fact that a digital image is processed with regard to the primary colors cyan, magenta, yellow and black and is converted to corresponding electric signals. These signals are then relayed to a thermal printer and converted into heat. The influence of heat causes the dye to sublime out of the donor layer of an ink ribbon (ink sheet) that is in contact with the receiving material so that it diffuses into the receiving layer.
A receiving material for thermal dye transfer usually consists of a base with a receiving layer applied to the front side of the base. The base may be a plastic film, e.g. polyester film, or a synthetic or resin-coated paper. The main component of the receiving layer is usually a thermoplastic resin with an affinity for the dye from the ink ribbon, such as polyester or acrylic resins. In addition to a receiving layer, other layers are frequently also applied to the front side of the base, such as barrier layers, separation layers, adhesion layer and protective layers.
High demands are made of a dye receiving material so that when they are met a high color density and image sharpness (line sharpness) in the transferred image should be assured. Various methods of optimizing the dye receiving material are known from the state of the art, e.g. by means of the support material or by applying various functional layers and/or by means of a specific choice and composition of the receiving layer.
U.S. Pat. No. 4,774,224 discloses a receiving material whose polyethylene coated paper base must have a surface roughness (Ra) of at most 7.5 .mu.inch.
In Japanese patent application No. 02 229 082 a receiving material is described whereby the polyethylene coated paper support material has a roughness value in the amount of 8 to 160 .mu.inch.
A disadvantage of both of these receiving materials is that not all polyethylene coated paper bases with the claimed roughness values in the polyethylene surface either below or above 7.5 .mu.inch guarantee good results with regard to the color density and image sharpness of the image transferred.
The present invention is based on the problem of developing a support material for thermal dye transfer processes which should assure a receiving material that will permit the production of images with a high resolution (line sharpness) and color density after application of a receiving layer.
This problem is solved by using a base paper with a surface roughness (Ra) of 4 .mu.m or less and applying a polyethylene coating to it in a maximum amount of 30 g/m.sup.2 to receive the support material for thermal dye transfer.
It has surprisingly been found that contrary to the claims made in the state of the art, the roughness of the base paper of a support material for thermal dye transfer processes does play a role in determining the quality of the image transmitted later. It has been found that maintaining a base paper roughness of &lt;4 .mu.m and applying the polyolefin coating in an amount of &lt;30 g/m.sup.2 make it possible to achieve a high color density and resolution (line sharpness) of the transmitted image. This is true not only for high gloss surfaces of the polyolefin coated support material (Ra&lt;0.2 .mu.m), but also for polyethylene surfaces with a greater surface roughness (&gt;0.2 .mu.m).
In a preferred embodiment of this invention, the roughness of the base paper is 2.5 .mu.m or less.
In another preferred embodiment, the amount of polyolefin coating applied is less than 15 g/m.sup.2.
The polyolefin coating may consist of high density polyethylene (HDPE) and/or low-density polyethylene (LDPE) or polypropylene. In addition, the polyolefin coating may contain pigments such as TiO.sub.2 and other additives.
According to this invention, the polyolefin coating contains at least 30% HDPE, preferably 40-80% HDPE.
This invention will be illustrated in greater detail with the help of the following examples.